Acoustic Surveys

At sea, much information about cetaceans can be gathered from acoustic surveys as well as visual surveys. In many cases MORE information can be gathered. A cetacean’s primary sense is that of sound, which they use to communicate, navigate and feed. Therefore we can learn much about them by listening to the sounds they make. Acoustic studies are particularly useful because cetaceans spend much of their time underwater and they can be difficult to observe, especially in rough conditions. By using acoustics we can still gather useful data even when conditions are too rough to spot an animal and also be used at night, a time when cetaceans are often more active.

To listen in on cetaceans HWDT uses what is called a hydrophone (an underwater microphone). We tow this from the aft of the boat during our surveys. On board Silurian we have two different systems for listening to cetaceans.

1) An audio system - This will detect anything within our human range of hearing. This means it will pick and allow us the listen to all dolphin species and also man made noise such as boat traffic, seal scarers and sonar. During surveys the audio hydrophone is listened to for 1 minute every 15 minutes. The boat is slowed, and if the engine is on, it is put out of gear to minimize background noise during listening. A digital recording is made of each stop directly onto the computer.

2) The second system we use is called a Porpoise hydrophone. Porpoise vocalize at a very high frequency (125 kHz). This frequency is beyond the range of human hearing and therefore we need specialist equipment to detect the high frequency echolocation clicks they make. This hydrophone is designed to pick up only these high frequency sounds.

Please click on the following to hear what some vocalisations of Hebridean species:

Below is a diagram of what a hydrophone looks like and how it is put together. The hydrophone consists of a single tube containing a stereo pair of audio-band hydrophones and a stereo pair of high frequency porpoise detector hydrophones. There is also a depth sensor at the end of the hydrophone. All these components are encased in a flexible plastic tube filled with ISOPAR paraffin oil and is towed from the stern of the boat on a 100m cable.

Hydrophone construction

Passive acoustic equipment layout

Battery / Power Supply Layout